1. Field of the Invention
The present invention relates to an electrode material for a secondary cell obtained by graphitization of a highly heat resistant polymer and having excellent cell performance, and to its production process.
2. Description of the Related Art
The popularity of personal computers, cellular telephones and similar devices is expected to continue to grow in the future accompanying the increasing utilization of information in society and, accompanying this growing popularity, secondary cells used as power sources for portable devices are being required to provide higher energy density and larger capacity. Lithium secondary cells, in particular, which use a non-aqueous electrolyte, have high cell voltage and high energy density.
Examples of secondary cells known in the prior art include button cells like that are described in Japanese Unexamined Patent Publication No. 63-58761, and coil cells like those described in Japanese Unexamined Patent Publication No. 4-161756. These cells are composed by coating and forming the positive and negative electrode materials on charge collectors comprising metal plates, installing a separator between both electrode plates, and immersing the assembly in an electrolyte.
In the cells of the prior art, graphite or amorphous carbon-based carbon black was used for the electrode conductor. However, these materials have the problem of the output density being too low. Cells having a high output density are required as the power sources for personal computers, cellular telephones, portable video equipment, electric automobiles and so forth. In addition, since a large amount of current is consumed when starting these devices, cell performance is required to withstand rapid discharge. In addition, in the case of large cells as well, an extremely large output is required when an electric automobile, for examples, starts, and a satisfactory rapid discharge performance is required. In the case of using graphite or amorphous carbon for the electrode, adequate discharge characteristics are not obtained. For example, discharge capacity decreases by about 20-30% when the discharge rate is raised from 0.325 C. (4 hour rate) to 2 C. (30 minute rate). In this manner, when the discharge rate is increased, cycle characteristics decrease correspondingly, thereby limiting the cell life. Among those factors that affect discharge characteristics, electron conductivity within the positive electrode is considered to be the dominant factor. Although the amount of graphite or amorphous carbon serving as the conductor could be increased in order to improve electron conductivity within the electrode, in this case, since the proportion of conductor that occupies the electrode increases, volume energy density, which is an important cell characteristic, ends up decreasing. Namely, improving electron conductivity within the electrode while maintaining a high volume energy density is important in terms of developing a practical cell.
With respect to lithium secondary cells, lithium metal, lightly graphitized carbon particles and highly graphitized carbon particles are used as negative electrode materials. In addition, there has also been active development of a low-temperature carbonized carbon negative electrode material in recent years.
Although lithium metal is able to realize a high charge/discharge capacity, due to its high reactivity, the capacity decreases as a result of reacting with solvent present in the electrolyte as the charge/discharge cycle progresses. In addition, since branched-form lithium metal forms easily, there is the problem of this branched-form lithium metal passing through the separator provided between the positive and negative electrodes causing a short-circuit. Although lightly graphitized carbon materials are characterized by having low reactivity with an electrolyte and being resistant to the formation of branched-form lithium metal, the charge/discharge capacity is typically low and, due to the low true density, it has the disadvantage of a low charge/discharge capacity per unit volume, thereby being unable to attain the realization of a secondary cell having high energy density.
In addition, although examples of low-temperature carbonized carbon negative electrode materials exhibiting remarkably high discharge capacities have been reported, these have significant practical problems including low initial charge/discharge efficiency, small charge/discharge capacity per unit volume due to the low specific gravity, high discharge potential, large hysteresis accompanying charging and discharging, and significant cycle deterioration.
On the other hand, highly graphitized carbon particles used for the negative electrode have a higher charge/discharge capacity in comparison with lightly graphitized carbon particles. Since they are also characterized by lower reactivity with an electrolyte and greater resistance to formation of branched-form lithium metal as compared with lithium metal, they have been studied actively in recent years for use as negative electrode materials, and expectations are being placed, in particular, on their use in the development of an electrode for secondary cells having excellent performance.
Lithium ions present in the compound between the graphite layers (negative electrode) are released from between carbon layers during discharge and return to the positive electrode. An interlayer compound is formed with lithium by using graphite for the negative electrode and charging electrochemically. It has been theoretically determined that the maximum capacity is achieved when in the state in which a single lithium atom is coordinated with respect to six carbon atoms (C6Li), and the discharge capacity of the negative electrode can be increased up to a maximum of 372 mAh/g-carbon.
In the case of highly graphitized carbon particles, since small, isotropic crystallites of the graphite structural crystals result in the end faces of the graphite crystals being exposed on the surface, it becomes easier for metal ions to coordinate, thereby making this preferable. In addition, since the higher the density of the graphite particles the greater the energy density, weight is reduced and charge/discharge capacity is increased. Consequently, coiled cells, for example, are typically produced by blending the graphite particle powder with a binder to form a paste, adhering this paste to a charge collector sheet, rolling and then coiling with the positive electrode and separator to form the cell.
The role of the binder here is to form the active substance into a paste, bind the active substance together, adhere the active substance to the charge collector sheet and contribute to the safety of the cell overall.
As an example of a secondary cell electrode, a secondary cell electrode is disclosed in Japanese Patent No. 2874999 which is obtained by bringing an alkali metal into electrical contact with a support composed of a mixture of a carbonaceous material, having a hydrogen/carbon atomic ratio of less than 0.315, a spacing distance d002 between surfaces (002) as determined by x-ray wide angle diffraction of 3.37 angstroms or more, and a crystallite size in the direction of the c axis (Lc) of 220 angstroms or less, and a polymer composition having alkali metal ion conductivity to cause the alkali metal loaded on the support as an active substance.
In addition, an electrode carbonaceous material for a secondary cell is disclosed in Japanese Patent No. 2856795 in which the hydrogen/carbon atomic ratio (H/C) is less than 0.315, the spacing distance between surfaces (002) as determined by X-ray wide angle diffraction is 3.39-3.75 angstroms, and the crystallite size (Lc) in the direction of the c axis is 5-150 angstroms.
In addition, a graphite material for a non-aqueous electrolyte secondary cell electrode is disclosed in Japanese Patent No. 2948097 in which the mean interlayer spacing distance d(002) between surfaces (002) as determined by X-ray diffraction is 0.3336-0.3350 nm, the crystallite size Lc(002) in the direction of the c axis is greater than 15 nm up to 50 nm, and the crystallite size La(110) in the direction of the a axis is 5-50 nm, wherein, a fine, optically anisotropic mosaic structure is confirmed by observation of polarizing micrographs at 1000xc3x97, and the actual dimension of the long axis reference of the anisotropic (mosaic) units that compose said optically anisotropic structure is 10 xcexcm or less.
However, since the cell performance of these known secondary cell electrodes of the prior art is still inadequate, there continues to be a need for a secondary cell electrode capable of attaining a high level of cell performance.
The object of the present invention is to provide an electrode material for a secondary cell having excellent performance with respect to charge/discharge cycle life and discharge voltage, and its production process.
In order to attain the above object of the present invention, the present invention provides an electrode material for a secondary cell comprising as its essential component a powder of graphite having crystallinity of 90% or more obtained by graphitizing a highly heat resistant polymer in an anaerobic atmosphere.
In addition, the present invention provides a process for the production of an electrode material for a secondary cell comprising heating and graphitizing a graphite precursor in which 0-10 wt % of a boron compound has been added to a highly heat resistant polymer to cause a powder of graphite, having a crystallinity of 90% or more, to be contained as its essential component.